Oedema—“swelling”—is a classic symptom of illness. The presence of oedema can be used as in indicator of various kinds of illness, such as heart and kidney disease, thrombosis, burns and lymph circulation disorder. For patients, oedema involves a number of inconveniences/problems, such as pain, change in centre of gravity leading to back problems and a general impairment to quality of life. Consequently, there is a need for measuring equipment to assess an oedema and measure how oedema is reduced by various treatments in a hospital. Oedema can generally be divided in to “pitting oedema” and “brawny oedema”. Brawny oedema arises when the fluid that causes the swelling remains inside the cells and not between them, as is the case with pitting oedema. Brawny oedema can also arise when the fluid causing the swelling is located between the cells but has coagulated due to the precipitation of fibrinogen. Often, pitting oedema and brawny oedema arise simultaneously in the same patient and it is known in literature that pitting oedema often turns into brawny oedema after a time.
Problems within Known Technology
Current methods of measuring oedema are principally designed to indirectly measure the volume of an oedematous (swollen) extremity and compare it with the volume of the corresponding normal extremity (volumetry). For example, such measurements for arm oedema are taken by placing the swollen arm into a cylinder full of water and then measuring the weight of the overflowing water. The same procedure is repeated with the normal arm and the difference in weight of the overflowing water gives a measure of the size of the oedema. Understandably, this method is very imprecise and small differences in the size of an oedema cannot be measured. Neither can the distribution of different types of oedema nor their location be determined with volumetry.
In recent years there have been a number of identometers/tonometers presented in the literature. These measure the occurrence of pitting oedema locally by e.g. depressing the skin at the site of the swelling and holding a depth of depression for a certain time of measurement, e.g. 20 seconds. The force required to maintain the depth of depression is registered and will become lower during the time of measurement depending more or less on how much of the free flowing oedema fluid that is displaced in the tissue under pressure.
One example of the tonometric method is the impression method that presses a circular measuring plate (e.g. D=15 mm) into the skin, normally 4 mm, and measures the force required during a period of 20 seconds. Lindahl and Omata (1995) made a comparison between the impression method and a tactile sensor with regard to the ability of the two instruments to detect pitting oedema.
The tonometric methods are limited in that they cannot measure brawny oedema as they assume the fluid in the oedema is free flowing.